Hydraulic fluid controls can be found in a variety of automotive applications such as automatic speed change transmissions as well as others. In these applications, it is often desirable to control the pressure of the hydraulic fluid, as seen by reference to U.S. Pat. No. 6,308,725 entitled “APPARATUS FOR CONTROLLING HYDRAULIC FLUID PRESSURE” issued to Lawlyes et al., assigned to the common assignee of the present invention. Lawlyes et al. disclose a smart actuator including a solenoid element and a pressure sensor element, both of which are in electrical communication with a remote control through a wire harness. Lawlyes et al. provide for remote pressure sensing of a solenoid output.
In the specific context of an automatic speed change power transmission, it is known to use electronic transmission control units that are configured to generate electrical signals that control solenoids resulting in the control of fluid flow as well as the pressure in a hydraulic fluid line. As known, the pressure in a hydraulic fluid line can be used to control various other elements in an automatic transmission system including for example a hydraulically-actuated clutch for the engagement of individual gears. By engaging various combinations of gears (e.g., planetary gears in a planetary gear transmission), an automatic transmission system accomplishes the same task as the shifting of gears in a manual transmission. Hydraulically-actuated clutches that are found in transmissions are typically used for engaging a pair of gears (e.g., a pair of rotating members, or alternatively, one rotating member and one non-rotating member) together such that when the clutch is applied, torque can be transmitted from one shaft to the other. Shift changes may also include switching three or more clutches on occasion for certain types of shifts, and herein references to two clutch type shifts could also include the multiple shifts.
An important operating aspect of a hydraulically operated clutch relates to the pressure build-up of the applied hydraulic fluid. In general, fluid flow at a certain applied pressure is sought to be controlled and varied to apply the clutch in order to obtain a desired engagement characteristic, principally with respect to timing and smoothness. It should be appreciated that if the timing of the engagement of one gear with the disengagement of another gear is not coordinately properly, overall shift performance may suffer. It is thus desirable and known in the art to control the clutch pressure.
In this regard, it is known to provide a linear solenoid to control the hydraulic fluid pressure to apply and/or release the clutch. In a linear solenoid, the amount of fluid at a controlled pressure can be varied by changing a solenoid control current. To achieve control of a system including a linear solenoid, it is known to employ software responsive to various inputs to control the current provided to the solenoid. However, the pressure output of a linear solenoid differs in dependence on the control current direction. More specifically, a transition from a relatively low control current to a relatively high control current is considered an increasing or “up” direction. Likewise, transitioning from a relatively high control current to a relatively low control current is considered a decreasing or “down” direction. A linear solenoid typically has a pressure-current (P-I) characteristic curve for each direction. Thus, for any desired control pressure value, the required solenoid current would generally depend on whether the direction of the transition to the desired control pressure is “up” or “down”.
One approach in the art for determining the required solenoid current to achieve a desired pressure provides for an “up” direction P-I table and a down direction P-I table. Then, during operation, the control determines what table to use based on current direction, and then extracts the required solenoid control current from the one selected P-I table. Further, under this conventional approach, calculating the required solenoid current may involve interpolation within the one selected table (e.g., where the desired control pressure is not exactly found in the data pairs in the table, the desired current may be interpolated from the nearest data pairs). One shortcoming of this approach however is that the data in each of the “up” and “down” P-I tables are generally most accurate for the specific conditions under which they were captured. Using the tables under different conditions results in undesired variance.
It is also known to use a linear solenoid in combination with other hydraulic fluid control devices. One example that may be used in an automatic speed change transmission may be a two-stage pressure control module, where the first stage is a linear solenoid (e.g., variable bleed solenoid—VBS) providing a pilot or control pressure in response to an input command (i.e., a solenoid control current) and where the second stage includes a spool valve configured to regulate a main fluid supply pressure in accordance with the control pressure. The spool valve outlet is coupled to a hydraulic circuit that includes a hydraulically-actuated clutch. In this configuration, a transmission control unit (TCU) or the like is responsible for shift control and develops a desired clutch pressure command based on available information. Then, the clutch pressure command is converted into a desired control pressure (i.e., in the 2-stage arrangement described above, the required pressure output from the linear solenoid to achieve the commanded clutch pressure). Finally, the control pressure is converted into the appropriate control current to feed to the solenoid. However, it has been observed that this type of hydraulic system may have differing gains depending on what gear the transmission is in. This multi-gain characteristic of the solenoid can cause undesirable variations in the actual clutch pressure relative to the commanded clutch pressure.
There is therefore a need for a method for transmission clutch control that minimizes or eliminates one or more of the problems set forth above.